Polymers coated on various substrates have many different uses ranging from protective coatings, corrosion resistant coatings, laminates, sealants and encapsulants to dielectric films for capacitors and the production of printed wiring boards.
Generally, in most uses, the adhesion of the polymer coating to the substrate upon which it is applied is of great importance. Very often, a particular polymer will not reproducibly adhere well when applied directly to a given type of substrate and therefore, is not suitable as a coating material for that substrate. It has previously been discovered, however, that the adhesion of many polymer-substrate pairs can be enhanced by the use by what has been termed a coupling agent. The coupling agent may be applied to the substrate surface or may be dispersed in the polymer. These coupling agents contain moieties which bond to the substrate as well as moieties which bond to the polymer. In some instances the same moiety may be responsible for bonding to both the substrate and the polymer. Various types of material have been reported as being suitable as coupling agents including, but not limited to silanes, Werner-type complexes, various metal organic compounds such as alkoxy titanates and other materials. For example, reference may be made to an article entitled "Adhesion Through Silane Coupling Agents" by Edwin P. Plueddemann, Journal of Adhesion, 2, 184 et seq. (1970). This reference indicates the use of silane coupling agents to couple various types of polymers including epoxies, polyesters, polystyrenes, phenolics, polypropylenes and polyamides, among others, to substrates such as glass, aluminum, steel, graphite and the like. The teachings of this article are incorporated herein by reference.
Although coupling agents have led to increased adhesion of both thermoplastic and thermosetting polymers to the various substrates as set forth above, there is still a need for greater improvement for many purposes. For example, while a silane coupling agent for an epoxy coating on steel may result in an adequate adhesion for corrosion resistant coatings under normal conditions, the adhesion may not be adequate under abnormal or severe conditions. Similarly, epoxy coatings over steel employing a silane coupling agent for printed circuit boards may not have adequate adhesion when immersed in various electroless plating baths especially at elevated temperatures or when the epoxy coating is applied at high temperatures such as by means of a fluidized bed coating process. There is also a need for improved bonding of metal sheathing in metal sheathed cable to the polymeric cable jacket material, usually polyethylene. Consequently, there is still a need for further improvements in coating adhesions for many applications and generally improved coating adhesion is desirable.